Championship leader Fabio Quartararo controlled his rising frustration on Friday to record the fastest time on the first day of practice for the Czech MotoGP.
The Frenchman, winner of both races so far this season for the Yamaha satellite team SRT, set his best time in the final seconds of the day's second session.
Before that he had struggled on a track with little grip and made no secret of his irritation.
His best time was 0.007 seconds faster than his Italian team-mate Franco Morbidelli.
"The positive thing is that we are very fast on one lap. But we have to work on the race pace," Quartararo said after a session raced in track temperatures pushing 40 degrees.
"I know where the problem is, so we can get to work."
Two riders who had surprisingly good days, Miguel Oliveira, a Portuguese rider at KTM-Tech3, and Frenchman Johann Zarco of Ducati-Avintia, were third and fourth.
Italian Francesco Bagnaia is out of the race and will need surgery on a broken tibia after falling off his Ducati-Pramac in the morning.
He will miss the race in Austria next weekend.
Spaniard Maverick Vinales, who is second in the standings, also suffered a minor crash early in the first session on his Yamaha.
He ended up fifth fastest in the second session.
"This morning I made a mistake, because I went a little bit wide and I hit the bumps, so I crashed," said Vinales.
"I hit my right shoulder, but it seems like everything is okay, so I'm quite happy about that."
Seven-time world champion in the elite class Valentino Rossi was down in 12th on his Yamaha while Ducati's Andrea Dovizioso was 15th.
Rossi is chasing his 200th podium place this weekend in MotoGP.
Japanese rider Takaaki Nakagami was fastest in the morning session.
World champion Marc Marquez is again missing after needing a second surgery on his broken right arm.
Combined first and second practice times ahead of the Czech MotoGP in Brno:
1. Fabio Quartararo (FRA/Yamaha-SRT) 1 min 56.502sec, 2. Franco Morbidelli (ITA/Yamaha-SRT) at 0.007sec, 3. Miguel Oliveira (POR/KTM-Tech3) 0.048, 4. Johann Zarco (FRA/Ducati-Avintia) 0.081, 5. Maverick Vinales (ESP/Yamaha) 0.166, 6. Joan Mir (ESP/Suzuki) 0.374, 7. Aleix Espargaro (ESP/Aprilia) 0.539, 8. Pol Espargaro (ESP/KTM) 0.557, 9. Takaaki Nakagami (JPN/Honda-LCR) 0.571, 10. Jack Miller (AUS/Ducati-Pramac) 0.607